248 MOLLUSCA FROM THE CRAG. 



Lenfftk, 1 inch. Height, f inch. 

 Locality. Red Crag, Sutton ? 



Mam. Crag, Bramerton. 



Clyde Beds, Bracklesham, Uddevalla. 



Recent, West Gothland, Britain, Mediterranean. 



This is a common shell in the Mammaliferous Crag in the neighbourhood of 

 Norwich. The cuneiform variety (cuneatd) differs only in so far as to have one 

 side a little more produced than is usual in the recent shell. The specimens are 

 always more or less deprived of their outer coating ; but in those best preserved, the 

 ridges upon the dorsal area may be distinctly seen. 



10. MACTRA OBTRUNCATA, S. Wood. Tab. XXIV, fig. 5, a, b. 



Spec. Char. Testa subaquilaterali, obtuse triangulari, obtruncatd ; antice angulatd ; 

 postice truncatd aut rotundatd; umbonibus subprominentibus ; area dorsali striatd ; dentibus 

 lateralibus rugosis. 



Shell slightly inequilateral, obtusely triangular, roundedly truncate ; anterior side 

 angular, posterior irregularly rounded ; umbones slightly prominent ; dorsal area 

 striated ; lateral teeth denticulated. 



Length, -| inch, nearly. Height, inch. 



Locality. Cor. Crag, Sutton. 

 Red Crag, Sutton. 



This shell is not rare either in the Red or Coralline Crag. It much resembles in 

 form the preceding species (M. subiruncata), but differs essentially in being reversed. 

 In the living species the angular or produced portion is on the side on which 

 the ligament is placed, and where the sinuated mark of the mantle is seen ; 

 while in this, the anterior side is angulated, though not much produced. It is a 

 thick and strong shell, somewhat tumid, with an obtuse ridge on each side, more 

 distinctly marked on the anterior, running from the umbo to the ventral margin, 

 within which it is slightly flattened. It has long remained in my cabinet, under the 

 name of M. subtruncata, imagining it to have been only a reversed variety, like Trophon 

 antiquum, var. contrarium, so common in the Red Crag. There appears, however, a 

 difference also in the form and direction of the sinus in the mantle-mark, more than 

 might be the result of a difference in the length of the siphonal sides of the two 

 shells. 



Mactra striata, Smith, 'Wern. Trans./ vol. viii, pi. 1, fig. 22, much resembles, and 

 may probably prove to be, this species, but I have not been able to obtain a specimen 

 for comparison ; and judging from the representation, it does not appear to have the 

 angular form on the anterior side as in our shell. Messrs. Forbes and Hanley referred 



